The present invention relates generally to holography and more particularly to aligning holographic images of Fourier region holograms.
Holograms store information based on the concept of a signal beam interfering with a reference beam at a holographic medium, which stores the interference pattern as a hologram. The signal beam may be reconstructed to generate the image of the hologram by illuminating the hologram with the reference beam at the same incident angle that the hologram was created.
One approach to store holograms is to record them on a disk, such as radially or spirally, as in typical compact disks. When the disk spins, many holograms can be stored on the disk. One can increase storage capacity through multiplexing. For example, each spot on the spiral track can store a number of holograms by steering the reference beam to different incident angles to illuminate that spot.
During readout, the disk, illuminated by the reference beam, can spin continuously. The signal beams re-generated are focused to get the holographic images, which are typically measured by a detector array.
When holographic images are read from the spinning disk, they could be defocused, shifted, rotated or distorted. These may be due to disk wobbling, disk decentration from its rotation axis, or deformation of the disk substrate. Such defects may degrade or introduce error to the signals measured by the detectors.
Unlike typical compact disk where every image consists of one pixel, in holographic storage, every image can include hundreds of thousands of pixels. It is not uncommon to have an array with hundreds of thousands of detector elements just to measure one hologram. All of the pixels, not only for one hologram, but for every hologram on the disk, should be in focus, and located at correct positions relative to the detector array. Correct alignment for so many pixels in the holograms is not an easy task.
One way to reduce the alignment problem is to over-sample the readout image. This can be done by using a detector array with many more detector elements than there are pixels in the holographic image. Then, based on error-correction codes, post processing may be able to re-create a defect-free image. However, this approach may not be able to solve de-focusing problem. Also, over-sampling requires a higher clock speed to retrieve data from the detector array. One way to operate at a slower clock rate is through parallelism. However, parallelism increases the size and the cost of the detector array. And sophisticated post processing may introduce extra bottleneck to the system's operating speed.
One approach to reduce cost and increase operating speed is to have a detector array whose pixels are one-to-one matched to the pixels of each holographic image. However, in order to be pixel matched for all the holograms, the holographic medium's mechanical position and orientation should be detected, with errors corrected.
As the disk spins, there are three degrees of freedom translationally, and three degrees of freedom rotationally. Relative to the detector array, during readout, the holograms may be shifted laterally, have changed its elevation, or tilted rotationally. Such mechanical position errors can be due to the disk being removable and the holder of the disk having limited precision. Removing the disk from and replacing it back into the holder may lead to alignment changes. Also, the disk is not perfectly flat, and is malleable. For instance, the manufacturing tolerance on bending in conventional CD-ROM disks is about 1.2 degrees. Thus, mis-aligning the holographic images to the detector array, and losing data in the images can easily occur.
It should have been apparent from the foregoing that there is still a need for methods and apparatus to properly track the disk to maintain alignment between the holographic images and the detector array, and to correctly read holographic images from holograms stored in disk format without introducing significant cost and bottleneck to the system.